1. Field of Invention
The present invention relates to probes and sensory guides for robot actuated tools, such as grippers, welding heads, positioning, measuring and counting devices, etc., that perform a control function without physically contacting the work piece being sensed.
2. Description of the Prior Art
Systems are well known wherein sensing devices locate a work piece or zone (work), and effect relative positioning between the work and a tool for performing an operation thereon, by comparative intelligence that is translated into a control function by which optimum relative positioning is sought to be achieved and maintained. Probes that physically contact the work and follow its contours to cause a tool to track through mechanical (including hydraulic and pneumatic) and electrical (including magnetic and electronic) linkages, are distinguished from contactless probes of present concern. The Zeewy et al. U.S. Pat. No. 3,883,956, exemplifies such physical probes as applied to a welding operation. A downwardly-biased finger probes the groove or joint to be welded. During relative movement between finger and groove, variations in the latter cause the finger to move, which produces voltages that are translated into movement of the responding welding torch in a duplicate pattern that, ideally, tracks the groove being welded. Such systems are marked by significant disadvantages: finger probes deteriorate due to weld spatter, and intense heat, consequent upon proximity to the welding zone; and in non-welding applications due to frictional wear, and can get in the way of the tool. More remote spacing greatly increases the complexity and cost of such systems when designed to offset the aggravated geometric problems. Good practice dictates that the probe be positioned as closely as possible to the tool, in a lead or advance location to the tool in order to minimize tracking errors without deleterious exposure to itself, or interference with the tool in performing the work to be done. Contactless probes which can be positioned well above the work commend themselves as best suited to achieve these ends.
Several forms of contactless probes are known to the prior art. Among these is the optical approach disclosed by Stanley, U.S. Pat. No. 3,009,049, employing a television monitor in a welding operation, which is subject to disadvantages of damage to the optical system from heat and spatter, as well as aberrations arising from flickering of the welding arc, glowing of the molten metal, and obscuration by flux in the seam ahead of the torch. Also such systems tend to be very complex, expensive, and intolerably slow in converting the television image into a position error signal, even in non-welding applications.
In another welding adaptation, Sullivan, U.S. Pat. No. 3,480,756, employs magnetic sensors to position a welding head with respect to a seam to be welded between which relative movement occurs. Magnetic systems lack the precision and general range of usefulness of the optical sensors, being, as they are, limited to certain magnetic materials, being insensitive to small changes or details in the welding path or work to be sensed unless too close to the work to preserve the probe's integrity, or so close as to interfere with the welding or tool itself.
In still another welding system, Iceland, U.S. Pat. No. 3,775,582, employs focused microwave energy with an interferometer to track a weld seam. Microwaves require a work surface that is made of conductive material in order for the waves to effectively reflect back to the receiver. Use of an interferometer presents the possibility of the system confusing the intended distance between the antenna and the work surface, and a distance one wave length longer or shorter. If a lower frequency which has a longer wave length is used, then the ability of a reasonable sized antenna to focus the waves is correspondingly reduced. Microwaves have the additional disadvantage of dispersing into the environment with attending problems of causing interference and raising the question of exposure to humans.
Erdman et al., U.S. Pat. No. 2,743,429, employs ultrasonic waves to measure the distance by echo time to a work piece, but can encounter the same possibility of confusing the distance with a distance one wave length longer or shorter, and provides no means for focusing the sonic energy so that it can be used as a probe to detect small details, objects or weld seams.
Davis et al., U.S. Pat. No. 4,012,588, and Whetstone et al., U.S. Pat. No. 3,821,469, employ a widely dispersed shock wave to locate a point in space but also do not employ any means to focus the energy so that the device can be used as a probe.
In my prior pending application, Ser. No. 155,944, filed June 3, 1980, for "Shockwave Probe", there is disclosed a probe that guides a welding torch or tool through shock wave signals beamed at the joint to be welded or part to be sensed, the echoes from which are translated into an electric signal which provides the robot a means to cause the welding torch or tool to locate and track the welding path or work without physical contact with the work piece. This is, essentialy, a shock wave method for producing an echo in which the elapsed time between sending the shock wave signal, and the reception of its echo, is compared with a pre-selected standard, and torch or tool controls are made to respond to comparative differences thus detected.
While ultrasonic echo devices exemplified by Erdman et al, "Automatic Positioning Device", identified above, afford many advantages over other prior art devices, some of which devices are discussed above, certain disadvantages have been revealed therein as well. Among these are limited accuracy, and sensitivity to extraneous noises. The limitation on accuracy is, in part, due to the characteristic in such an acoustical system, by definition, to detect acoustical waves that lie beyond an initial oscillatory cycle, thus, introducing uncertainty and the possibility of error up to one wavelength. The first cycle received starts from zero amplitude, with considerable interference from extraneous noise, and increases in amplitude within the next few cycles, with the result that the accurate location of any point on the first cycle is obscured. This problem can not be solved through the use of higher frequencies, due to the fact that the ambient atmosphere readily absorbs frequencies not greatly exceeding 100 KHz.